1. Field of the Invention
This invention relates, generally, to a method and apparatus for article inspection and more particularly to an optical scheme for high speed inspection of densely packed areas of interest, utilizing a plurality of optical fibers, a light source and a rotating mirror.
2. Related Art
The manufacture of semiconductor apparatus increasingly moves to more and more dense packaging. Accordingly, this means that although more devices are associated with any component or assembly and hence the total number of packages/devices decreases, the value of each package/device increases.
Further, due to this increased density, it is more and more difficult the produce quality products. One of the contributing factors is that only a small number of defects, any one of which may occur in a variety of steps and processes, may cause the entire end product or component to be defective. Therefore, in effect, due to this increasing density, a smaller number of defects per step, when cumulatively taken in total over a, now more dense product, may result in the production of scrap.
It is therefore more and more important not only to perform each step/process correctly, but it is even more important to inspect each step/process or series thereof for such defects.
This has the effect of placing an increasing burden on the whole process. As these devices and the like become more dense, the complexity of testing increases.
This problem is particularly evident in the production and testing of multilayer ceramics (MLC). A MLC performs essentially like a multitude of layered circuit boards. Each layer, in the art referred to as "green sheets" has a plurality of apertures or "vias" therein. During manufacture, these vias must be filled with an electrically conductive paste. Thereafter, these filled vias are overlayed onto an adjacent green sheet, also containing vias. In this fashion a "sandwich" is formed having conductive circuit paths therethrough and therein so as to form an extremely dense MLC "brick".
By way of illustration, a typical layer in a glass ceramic product may have 50,000-200,000 (and theoretically more) vias, over an active area of 160 mm.times.160 mm (slightly more than 6 inches square). Due to such densities, and driven by the need to inspect each via, visual inspection is extremely difficult. Such inspections should ideally consider or check for full or partially filled vias, overfilled vias, registration of the vias and the like.
Unfortunately, it has heretofore been not only difficult to check for such parameters, but the time to do so has been prohibitively long. Some devices, readily available on the market may take minutes to accomplish such testing, with only a moderate degree of reliability. Should it become necessary to inspect more than a few number of green sheets in any given day, the time required for testing simply would not allow for testing of an entire daily production run.
Therefore, the choice is for the usage of untested product, or the hold-up of production until testing can be done. However, in the economic manufacturing environment as is present today, either choice is not a competitive solution.
A number of schemes have been disclosed which have been attempted. Such methods include an article by Ninomiya, et. al. entitled "Automatic 21/2D Shape Inspection System for Via-Hole Fillings of Green Sheets By Shadow Image Analysis, IEEE, 1989, Ch2750-8/89, pps 515-520. There, a system is disclosed which uses shadow imaging theory for via inspection. However, this scheme is not practical for large scale, dense green sheets in that the front-end optics are simply inadequate for precise, fast testing. In a sample 160.times.160 mm greensheet, it has been judged that the Ninomiya method would require over 21/2 minutes for testing, produce approximately, 0.6 false alarms per sheet, would require an extremely high power light source, and would operate at a very low pixel rate.
A second solution was suggested in U.S. Pat. No. 4,737,650, entitled "Inspection Apparatus" and issued Apr. 12, 1988 to West. While this reference describes a scanning apparatus, it does not address the means to handle large amounts of pixel data which this would create.
U.S. Pat. No. 4,421,410, entitled "Method and Apparatus for Inspecting Printed Wiring Boards" and issued Dec. 29, 1983 to Karasaki is limited in the size/number of defects which are detectable.
U.S. Pat. No. 4,162,126, entitled "Surface Detect Test Apparatus", issued Jul. 24, 1979 to Nakagawa, et. al. is for testing only particular types of surface defects and by virtue of its scheme, is simply too slow for high density green sheets.
British Patent 1,441,386, entitled "Scanning Apparatus", Published Jun. 30, 1976 discloses merely an optical scheme for a moving/oscillating mirror and as such is incomplete and inherently slow.
A publication entitled "Three Laser Scanning Instruments for Automatic Surface Inspection" for British Steel Corporation, pps 102-110, by R. N. West similarly lacks a scheme for handling large amounts of data collected.
U.S. Pat. No. 3,750,189, entitled "Light Scanning and Printing System" issued Jul. 31, 1973 to Fleischer discloses an optical system essentially for use in present day facsimile machines and hence lacks the accuracy required.
U.S Pat. No. 4,681,441, entitled "Method for Surface Testing", issued Jul. 21, 1987 to Wagner discloses a fiber optic collection scheme which is deficient in that it lacks the speed required for fast optical scanning given the required resolution.
U.S. Pat. No. 4,738,533, entitled "Plywood Surface Defect Detecting Head", issued Apr. 19, 1988 to Iwamoto discloses a course system of optical inspection.
U.S. Pat. Nos. 3,806,252 entitled "Hole Measurer", issued Apr. 23, 1974 to Harris, et. al. and 3,636,363, entitled "Device for Measuring Hole Sizes", issued Jan. 18, 1972 to Beeman, et. al. describe only mechanisms which are not fast AND are not sufficiently accurate for measuring small aperture diameters.
U.K. Patent Application 2,117,897, entitled "Detecting Surface Defects", filed Apr. 3, 1983 effectively provides only a "1" dimensional look of a surface and hence is simply not suitable for Green Sheets.